This invention relates generally to waste heat recovery systems and, more particularly, to a method and apparatus for detecting a low charge of working fluid therein.
As used herein, the term “waste heat recovery system” includes distributed power generation systems, such as organic rankine cycle power plants, which use a pump to transport the refrigerant in the system. A typical waste heat recovery system includes an evaporator, a turbine, a condenser, and a pump. The evaporator absorbs heat from heat source, which causes the refrigerant in the evaporator to boil off into a gaseous state. The refrigerant flows from the evaporator outlet to a turbine, in which the refrigerant is expanded to a low pressure condition. From the turbine the refrigerant circulates first to a condenser, where the refrigerant is cooled to a liquid state, and then to a pump, after which the pressure is raised. From the pump, the refrigerant circulates back to the evaporator, and the cycle is repeated. Efficient and safe operation of the system requires that proper refrigerant circulation and an appropriate refrigerant charge level be maintained.
In respect to the need for maintaining a proper refrigerant circulation at the proper conditions, a superheat controller can be applied to sense the various conditions of refrigerant passing through the turbine and to responsively vary the speed of the pump in order to maintain the desired refrigerant conditions. Those conditions include maximum pressure and temperature levels as well as a minimum superheat level.
Separate from the issue of maintaining proper refrigerant conditions is that of maintaining a proper refrigerant charge. A low refrigerant charge condition may result in the pump reaching its maximum operating speed, after which point the thermal dynamic loop is no longer in control thereby causing serious problems, such as damage to the turbine due to reduced cooling capability because of high superheat. The problems also include pump damage due to cavitations, reduction in plant efficiency, and reliability. In addition, low charge condition also means undesirable emission of refrigerant gases into the environment.
Most known low-charge detection systems have been developed for use in air conditioning systems. These systems use a combination of two or more temperature sensors, pressure switches, or pressure transducers. They tend to be particularly unreliable for the application in waste heat recovery systems.